MIAMI May 22 (Reuters) - David Beckham will be thrust into a political campaign to win backing for his Major League Soccer team in Miami with a referendum in November on a new stadium proposal.

On Thursday, the former England, Manchester United and Real Madrid midfielder's business partners unveiled their plan for a stadium on downtown Miami's waterfront after their original site, at the city's port, ran into political opposition.

The proposed 20,000 to 25,000-seat stadium would be part of an expanded park area close to the Miami Heat basketball arena.

Beckham's business group, Miami Beckham United, need to ink a stadium deal before they can win final approval for the franchise from MLS, the top professional division in the United States. Beckham also played with Los Angeles Galaxy in MLS for five years until the end of 2012.

Beckham was not present at the launch but said in a statement that the park would have public benefits and bring a professional soccer team to downtown Miami for the first time.

"I've always said that our team will be the people's club - another downtown amenity that makes the entire Miami-Dade community proud," said Beckham.

While the plan has the conceptual backing of the mayors of the City of Miami and Miami-Dade County, the proposal will go to a referendum, likely to be held alongside mid-term elections in November.

Bolivian billionaire Marcelo Claure, who along with entertainment entrepeneur Simon Fuller, is Beckham's partner in the venture, said Beckham would be out in the community seeking votes.

"You will see that a lot. You will see that very soon. David says this is one of the most important projects of his life. If you look at his career, David has been a success on and off the field, there are no failures," he said.

Claure said that while the focus was on securing a stadium, the trio had no shortage of interest from potential investors.

"Right now there are three partners for the team but there is always interest. One thing I've found since partnering with David is that everybody wants to be involved. From China to Argentina we have had hundreds and hundreds of calls," he said. (Editing by David Adams and Grant McCool)